The present invention relates to a synchronous motor.
Generally, a synchronous motor requires a starting means for accelerating its rotor to a rotating speed of rotating magnetic fields produced by stator windings, that is, approximately to a synchronous speed, and a means for effecting DC excitation of rotor windings for the normal running operation.
An induction synchronous motor has been devised for omitting a starting means and having the synchronous motor itself to posses a starting torque so that the motor can first start as an induction motor with the rotor windings being short-circuited whereby no special means for starting the motor is required. However, such a motor requires brushes since the rotor windings must be excited by DC current for synchronous operation of the motor. When the rotating speed of the rotor approaches the synchronous speed, the short-circuiting of the rotor windings is released so that the DC current is allowed to flow to the rotor windings through the brushes from the external DC current power source thereby producing magnetic poles in the rotor. These magnetic poles are attracted by the rotating magnetic fields produced by the stator windings so that the rotor is caused to be rotated at a synchronous speed. The brushes require maintenance checks resulting in extra costs for maintenance so that the development of a synchronous motor having a brushless configuration is strongly desired.
As conventional synchronous motors having a brushless configuration, there are motors of permanent magnet types or reluctance types but these are all limited to small capacity motors because induction starting is not possible and the starting torque is small. Such motors of an inductor type have disadvantages because magnetic passages therein are complex resulting in an increase in the size of the motors. The same is true in the motors using therein an AC exciter and a rotary rectifier means. A three-phase synchronous motor of a brushless type utilizing a harmonic magnetic field through a square wave voltage of an invertor by having a diode connected to the rotor windings has a disadvantage in that the magnetic magnetizing power of the rotor is insufficient and the output is not strong enough.
Further, Japanese Patent Application Kokoku No. Sho 54(1979)-34,124 discloses an arrangement wherein the starting is effected according to the induction motor theory and the synchronous operation is effected by having the DC magnetic fields formed in the axial direction thereby causing the rotor cores to form magnetic poles. This has a drawback in that the torque produced is asymmetrical with respect to the rotary axis, thereby causing the axis to vibrate.
Also, Japanese Patent Application Kokoku No. Sho 61(1986)-1,992 discloses an arrangement wherein there are two rotating magnetic fields of four poles and eight poles which do not interfere with each other and two phases of the three-phase rotor windings are for the synchronous operation and the remaining one phase is used for the starting operation with the rotor windings of this one phase being short-circuitted. However, a disadvantage therein is that the starting torque may tend to be small.